This is what I am trying to wrestle with. I know that I tend to not talk and withhold information that is probably not in anyone's best interest. I'm working on learning how to share more of myself with others and be less paranoid. But this issue with H, is not paranoia...it real!

I'm trying to find a middle ground.

When I think about Sittingatwatersedge situation...I immediately identified with person A having no control in what is being disclosed about them. I guess if I were person B, before I started confiding in a third party, I would tell person A I'm going to do it. Or at least clearly indicate that I strongly feel disclosure is important and that I cannot be trusted to keep my mouth shut.
Also, I think if I where sittingatwatersedge who is being pulled into a the fray... I would tell person B, "Hey before you go telling me private stuff... I want you to know that if I think I should talk to my brother or sister about what you tell me... I am going to." In other words, there are no secrets between the three of us after that point. Or maybe another way to handle it is to tell B, it sounds like you have something going on, I'm willing to listen but A has to be included in the conversation too.